Second Stalk of ATP Synthase

ATP synthase consists of two portions, F 1 and F o, connected by two stalks: a central rotor stalk containing γ and ε subunits and a peripheral, second stalk formed by δ and two copies of F o b subunits. The second stalk is expected to keep the stator subunits from spinning along with the rotor. We isolated a TF 1 - b ′ 2 complex (α 3 β 3 γδε b ′ 2 ) of a thermophilic Bacillus PS3, in which b ′ was a truncated cytoplasmic fragment of F o b subunit, and introduced a cysteine at its N terminus ( b c′). Association of b ′ 2 or bc ′ 2 with TF 1 did not have significant effect on ATPase activity. A disulfide bond between the introduced cysteine of bc ′ and cysteine 109 of γ subunit was readily formed, and this cross-link caused inactivation of ATPase. This implies that F o b subunit bound to stator subunits of F 1 with enough strength to resist rotation of γ subunit and to prevent catalysis. Contrary to this apparent tight binding, some detergents such as lauryldodecylamine oxide tend to cause release of b ′ 2 from TF 1.